Most present-day graphical computer user interfaces (GUIs) are designed using visual components of a fixed spatial scale. However, it was recognized from the birth of the field of computer graphics that visual components could be represented and manipulated in such a way that they do not have a fixed spatial scale on the display, but can be zoomed in or out. The desirability of zoomable components is obvious in many application domains; to name only a few: viewing maps, browsing through large heterogeneous text layouts such as newspapers, viewing albums of digital photographs, and working with visualizations of large data sets. Even when viewing ordinary documents, such as spreadsheets and reports, it is often useful to be able to glance at a document overview, and then zoom in on an area of interest. Many modern computer applications include zoomable components, such as Microsoft® Word® and other Office® products (Zoom under the View menu), Adobe® Photoshop®, Adobe® Acrobat®, QuarkXPress®, etc. In most cases, these applications allow zooming in and out of documents, but not necessarily zooming in and out of the visual components of the applications themselves. Further, zooming is normally a peripheral aspect of the user's interaction with the software, and the zoom setting is only modified occasionally. Although continuous panning over a document is standard (i.e., using scrollbars or the cursor to translate the viewed document left, right, up or down), the ability to zoom and pan continuously in a user-friendly manner is absent from prior art systems.
First, we set forth several definitions. A display is the device or devices used to output rendered imagery to the user. A frame buffer is used to dynamically represent the contents of at least a portion of the display. Display refresh rate is the rate at which the physical display, or portion thereof, is refreshed using the contents of the frame buffer. A frame buffer's frame rate is the rate at which the frame buffer is updated.
For example, in a typical personal computer, the display refresh rate is 60–90 Hz. Most digital video, for example, has a frame rate of 24–30 Hz. Thus, each frame of digital video will actually be displayed at least twice as the display is refreshed. Plural frame buffers may be utilized at different frame rates and thus be displayed substantially simultaneously on the same display. This would occur, for example, when two digital videos with different frame rates were being played on the same display, in different windows.
One problem with zooming user interfaces (ZUI) is that the visual content has to be displayed at different resolutions as the user zooms. The ideal solution to this problem would be to display, in every consecutive frame, an exact and newly computed image based on the underlying visual content. The problem with such an approach is that the exact recalculation of each resolution of the visual content in real time as the user zooms is computationally impractical if the underlying visual content is complex.
As a result of the foregoing, many prior art ZUI systems use a plurality of precomputed images, each being a representation of the same visual content but at different resolutions. We term each of those different precomputed images a Level of Detail (LOD). The complete set of LODs, organized conceptually as a stack of images of decreasing resolution, is termed the LOD pyramid—see FIG. 1. In such prior systems, as zooming occurs, the system interpolates between the LODs and displays a resulting image at a desired resolution. While this approach solves the computational issue, it displays a final compromised image that is often blurred and unrealistic, and often involves loss of information due to the fact that it represents interpolation of different LODs. These interpolation errors are especially noticeable when the user stops zooming and has the opportunity to view a still image at a chosen resolution which does not precisely match the resolution of any of the LODs.
Another problem with interpolating between precomputed LODs is that this approach typically treats vector data in the same way as photographic or image data. Vector data, such as blueprints or line drawings, are displayed by processing a set of abstract instructions using a rendering algorithm, which can render lines, curves and other primitive shapes at any desired resolution. Text rendered using scalable fonts is an important special case of vector data. Image or photographic data (including text rendered using bitmapped fonts) are not so generated, but must be displayed either by interpolation between precomputed LODs or by resampling an original image. We refer to the latter herein as nonvector data.
Prior art systems that use rendering algorithms to redisplay vector data at a new resolution for each frame during a zoom sequence must restrict themselves to simple vector drawings only in order to achieve interactive frame rates. On the other hand, prior art systems that precompute LODs for vector data and interpolate between them, as for nonvector data, suffer from markedly degraded visual quality, as the sharp edges inherent in most vector data renditions are particularly sensitive to interpolation error. This degradation is usually unacceptable for textual content, which is a special case of vector data.
It is an object of the invention to create a ZUI that replicates the zooming effect a user would see if he or she actually had viewed a physical object and moved it closer to himself or herself.
It is an object of the invention to create a ZUI that displays images at an appropriate resolution but which avoids or diminishes the interpolation errors in the final displayed image. A further object of the present invention is to allow the user to zoom arbitrarily far in on vector content while maintaining a crisp, unblurred view of the content and maintaining interactive frame rates.
A further object of the present invention is to allow the user to zoom arbitrarily far out to get an overview of complex vectorial content, while both preserving the overall appearance of the content and maintaining interactive frame rates.
A further object of the present invention is to diminish the user's perception of transitions between LODs or rendition qualities during interaction.
A further object of the present invention is to allow the graceful degradation of image quality by blurring when information ordinarily needed to render portions of the image is as yet incomplete.
A further object of the present invention is to gradually increase image quality by bringing it into sharper focus as more complete information needed to render portions of the image becomes available.
It is an object of the invention to optimally and independently render both vector and nonvector data.
These and other objects of the present invention will become apparent to those skilled in the art from a review of the specification that follows.